It is frequently necessary for men and machines to work in the vicinity of hazardous electric fields. For example, mobile cranes engaged in construction or maintenance of power distribution systems. Federal agencies and some states have regulations that such equipment should not be operated within 10 feet of such energized power lines. However, operator misjudgement, forgetfulness, equipment malfunction, etc. sometimes allows equipment to come into contact with such power lines. While these accidents are rare, and constitute a small percentage of total crane incidents, they contribute to a large percentage of fatalities.
Several types of sensing or monitoring equipment have been devised to help prevent such accidents. One type employs a computer model of the relationship of the crane and its boom and jibs to the power line. This requires all moveable portions of the crane to be equipped with the appropriate sensors to relay the positions of the crane's components to the computer. Also, the exact geometry of the terrain and power lines must be accurately known and entered into the computer. Not only is this expensive, but this solution also requires a large degree of cooperation and attention of the crane operator.
Other systems employ sensing of the electrostatic or electromagnetic fields around the power lines to provide an alarm when the crane penetrates the field to a preset distance,
Sensing of the electromagnetic field is simple, but is not practical because the magnetic field is produced by current flow in the lines. This current flow can change through wide values from moment to moment as load conditions vary.
Electrostatic proximity detectors now in use generally employ either a long sensing wire stretched along the crane boom or a point sensor mounted at some point on the boom. The distributed sensor will provide coverage along the side of the boom facing the electric field but will be "shadowed" on the other sides of the boom. Significant variations in sensitivity will be introduced by changing the length of the boom or by changing the orientation of the boom in relation to the power lines.
Tests have been conducted with distributed wire antennas and with a point contact probe mounted on a crane boom. If a crane is restricted to limited motions near an energized power line, then either system offers some warning as to hazardous approach. If, however, the crane were granted full mobility, such as by changing the orientation of the boom from perpendicular to horizontal with respect to the power line, or if the crane boom were moved from under the power line to over the power line, then the protection offered would change drastically. This occurs since the sensitivity of the antenna is affected by the orientation of and shielding by the boom, and distortion of the field by the cab, etc.
If multiple sensors are utilized, then the changes due to orientation and shadowing can by minimized. However, wiring of each sensor to the detector is expensive and difficult since some cranes have telescoping booms or jibs, and take-up reels are necessary to wind up the sensor wires. Additionally, the end of some booms (the end most in need of protection) usually has pulleys for the crane cable and is not suitable for the placement of sensor antennas.